Forms of hikite

A question. If the conventional hikite hand often has a wrist held by it, and the palm up hikite across the chest (as seen when performing shuto for example) often holds an arm, what is the function of a vertical open hand in the centre of the chest as seen in kata such as Sepai and Saifa?

Its been a while since I've seen either of these kata, but I use hikite to off balance the opponent by grabbing in some kind. Hikite is more of a principle in general and each move may have variations on it. Think for example Heian Nidan opening three moves - It shows you three ways of using hikite whilst attacking with the other hand.

If you can slow this clip down you might get a few ideas http://www.youtube.com/watch?v=MmExIWEglSo hint about the 0.40 seconds mark. Gets back to Ian's principle of dealing with the arms to get to the head. In this case grabbing it for a take down

I sometimes use a soete as an initial parry, followed by a strike. That is in my opinion a variation on the original hikite principle of pulling/grabbing/controlling. The biggest difference in this application is the direction you guide the opponents hand to.
A short example: a migi-choku-tsuki (rh) comes somewhere slightly under the chudan 'point'. I use my hidari-soete (lh) to parry it towards my migi-hikite position, followed by a migi-gedanbarai/gedan-kote-uchi. But by that time I have hidari-hikite, ready to deliver a hidari-zuki of some kind (shotei, kensei, nukite, etc.).

I'm not familiar with the kata mentioned. But after seeing video's, I can imagine applying something like I mentioned above.

Since it is the first movement (after yoi) in the kata, it should not be interpreted as higite but as a defense, I suggest.. The other arm could either hit with nukite or lock or throw. Jock´s example is from the outside but you can imagine doing it from the inside. But first defense, then grab, then lock and throw (one movement, different phases).

In sepai one orthodox interpretation is similar. After having been grabbed at the wrist (with one or two hands) you free yourself by going towards the opponent and work with your elbow. Goju ryu and shito ryu offer variants, and the application is accordingly a bit different. Then you hit down his arms (teisho-otoshi?), eventually grab and simultaneously hit uraken to the face (make the way free for the counter-attack and"bind" the arm that it cannot block anymore).

higite in general reminds you that not only one arm works and the other sleeps. It is no matter of debate that in application higite should usually not be at the hip since you would be open for the next attack.

Perhaps higite comes from weapon fighting. If both hands grab a sword or a bo, both arms have to work simultaneously for obvious reasons and this increases energy!